This is from JE pistons
This brings us to how the engine should be run after initial startup. The ideal combination is to tune the engine so that it starts on the first or second revolution. For carbureted engines, this means pre-filling the fuel bowls and accurately setting the initial timing. As soon as the engine starts, immediately bring it up to an above-idle speed. According to JE’s Senior Technical Account Manager Alan Stevenson, “You don’t want to break-in an engine at idle. You want to keep the rpm above 1,500 and vary the speed continuously for about the first 20 minutes.” After bringing the engine up to normalized coolant and oil temperature, put the engine under load. The cylinder pressure from 50 to 75 percent and eventually to 100 percent load will place additional pressure on the back side of the rings which will quickly establish the proper wear pattern for seating. With today’s rings, especially moly-faced versions, this can be achieved in a very short period of time and certainly within 20 to 30 miles of street driving. In WOT dyno testing, likely the rings are seated by the end of the first few runs.
Break-in oil should also only be used for initial engine run-in and then changed along with the filter and replaced with the engine oil you intend to run. On a street engine, this would mean less than 100 miles. Changing the break-in oil removes the impurities that will be present in the oil from the break-in period. This is especially true with race engines that use less restrictive filters. JE also recommends avoiding synthetics during break-in in order to take full advantage of establishing wear patterns. Synthetics sometimes can do too good a job of reducing friction so that the rings cannot seat properly.
In my early days, i used to rebuild the engine , fill cheapish oil, take it for a 300km drive and vary the speeds and loads constantly, drain oil, refill with good oil and send it full tilt from then onwards as i was very impatient person in my twenties.
